Color cathode ray tube

ABSTRACT

A color cathode ray tube is constituted of a glass bulb which includes a panel portion having a faceplate portion with a phosphor screen on an inner surface thereof, a neck portion in which an electron gun is mounted, and a funnel portion which connects the panel portion and the neck portion. Assuming a thickness of a screen effective area of the faceplate portion along the tube axis direction as tc at a central section thereof, as tx at a peripheral section thereof along the major axis direction, and as td at a peripheral section thereof along the diagonal direction, the cathode ray tube has the relationship tc&gt;td&gt;tx.

The present invention relates to a color cathode ray tube of the typewhich is used in a color television set for personal use and to a colordisplay monitor for an information terminal; and, more particularly, theinvention relates to a color cathode ray tube which has a faceplatepanel of improved curved shape.

BACKGROUND OF THE INVENTION

The glass envelope of a cathode ray tube generally comprises a panelportion having a curved faceplate, a neck portion with a reduceddiameter, and an approximately funnel-shaped portion which connects thepanel portion and the neck portion. The cathode ray tube furtherincludes a phosphor screen formed over the inner surface of thefaceplate, an electron gun installed inside of the neck portion, and adeflection yoke mounted on an outer periphery of the funnel portion.Here, the glass envelope of the cathode ray tube has a near vacuum inits interior, with atmospheric pressure being impressed on it, outerside at all times, so that the glass envelope is required to have amechanical strength that is higher than a predetermined level. For thisreason, various parts of the glass envelope are formed to a sufficientthicknesses to be able to provide the corresponding required mechanicalstrengths.

In a known cathode ray tube, the faceplate of the glass envelopenormally has a construction in which the peripheral section of thefaceplate is made thicker than the central section of the faceplate.FIG. 11 is a cross-sectional view showing one example of theconstitution of the faceplate portion of a glass envelope of the typeused in a known cathode ray tube.

In FIG. 11, numeral 31 indicates a faceplate, numeral 31(1) indicates aninner surface of the faceplate, numeral 31(2) indicates an outer surfaceof the faceplate, tpc indicates the thickness of a central section ofthe faceplate 31, tpa indicates the thickness of a peripheral section ofthe faceplate 31, Rpi indicates the radius of curvature of the innersurface 31(1) of the faceplate with a deflection center point O of theelectron beam taken as its center, and Rpo indicates the radius ofcurvature of the outer surface 31(2) of the faceplate with thedeflection center point O of the electron beam taken as its center.

As shown in FIG. 11, the faceplate 31 is constructed such that thethickness tpa of the peripheral section is greater than the thicknesstpc of the central section so as to provide the required mechanicalstrength as described above. As a result, the radius of curvature Rpi ofthe inner surface 31(1) of the faceplate is smaller than the radius ofcurvature Rpo of the outer surface 31(2) of the faceplate, that is,tpc<tpa and Rpi<Rpo.

In the above-described known cathode ray tube, the thickness tpc of thecentral section of the faceplate 31 is thin and the thickness tpa of theperipheral section is thick so that when an image is displayed on thephosphor screen formed on the inner surface of the faceplate 31, lightirradiated outwardly from the phosphor screen through the faceplate 31becomes attenuated more at the peripheral section of the faceplate 31having the large thickness tpa than at the central section of thefaceplate 31 having the small thickness tpc. That is, if we let Tpcstand for the light transmittivity at the central section of thefaceplate 31 and Tpa for a light transmittivity at the peripheralsection, then Tpc>Tpa and the brightness of the displayed image is lowerat the peripheral section of the faceplate 31 than at the centralsection of the faceplate 31, thus giving rise to a problem that thebrightness of a displayed image cannot be ensured at a sufficient levelat the peripheral section. The brightness at the peripheral section isfurther degraded by the fact that the weight of the phosphor is smallerat the peripheral section of the screen than at the central section ofthe screen.

To correct the brightness of the displayed image at the peripheralsection of the faceplate 31 so that it will match the brightness of thedisplayed image at the central section, when the brightness of thedisplayed image at the peripheral section of the faceplate 31 is loweredcompared with the brightness of the displayed image at the centralsection, the intensity of the electron beam projected onto theperipheral section of the phosphor screen needs to be stronger than theintensity of the electron beam projected onto the central section of thephosphor screen. Such a means for correcting the electron beamintensity, however, cannot be easily obtained.

To cope with such a problem, an applicant of the present invention filedan application (Japanese Laid-open Patent Publication 18547/1998), inwhich the curved shape of the faceplate of a cathode ray tube is formedsuch that the radius of curvature of the inner surface thereof is largerthan the radius of curvature of the outer surface, and the thickness ofthe peripheral section is made thin compared with the thickness of thecentral section of the faceplate, so that the brightness of a displayedimage at the peripheral section of the faceplate is increased, and thebrightness of the displayed image at the peripheral section isapproximated to the brightness of the displayed image at the centralsection.

However, in the above-mentioned cathode ray tube, since the radius ofcurvature of the inner surface of the faceplate is larger than that ofthe outer surface of the faceplate, the radius of curvature of a shadowmask, which is arranged to face the phosphor screen formed on the innersurface of the faceplate in an opposed manner and performs colorselection, becomes large as a whole. In a cathode ray tube having such aconstitution, since the molding retaining strength of the curved surfaceof the shadow mask becomes weak, the curved surface is easily deformed,so that the electron beams which pass through apertures formed in theshadow mask do not normally impinge on the phosphor screen (making itimpossible to perform normal color selection) whereby there arises a newproblem in that a deterioration of the color purity of the displayedimage is induced.

Particularly, at the peripheral section of the screen along the minoraxis direction, the shadow mask is liable to be easily influenced byvibrations or an impact at the time of dropping; and, hence, when theradius of curvature of the shadow mask is increased, the mechanicalstrength of the curved surface becomes weak, so that the curved surfaceis liable to be easily deformed in that area.

Further, the inside of the glass bulb is approximately in a vacuum stateand atmospheric pressure is always applied to the outside of the glassbulb. Accordingly, in view of the geometric structure of the glass bulbfor a cathode ray tube, a stress strain is liable to be concentrated onthe peripheral section of the screen, along the minor axis direction inparticularly; and, hence, when the glass thickness of the peripheralsection along the minor axis direction becomes thin by increasing theradius of curvature of the inner surface of the faceplate, themechanical strength of the glass bulb (panel portion) becomes weak inthat area.

Further, at the peripheral section of the screen along the diagonaldirection, the distance from the center of the screen becomes longest;and, hence, when the radius of curvature of the shadow mask becomeslarge, the molding retaining strength of the curved surface isdecreased, so that the curved surfaces are liable to be easily deformedbetween the center and the peripheral section along the diagonaldirection.

The present invention has been made to solve the foregoing problems, andit is an object of the present invention to provide a color cathode raytube in which the brightness of a displayed image at a peripheralsection of a faceplate will match the brightness of a displayed image ata central section of the faceplate using means having a simpleconstitution and, at the same time, can prevent the degradation of thecolor purity of the displayed image and the lowering of the mechanicalstrength of the glass bulb.

SUMMARY OF THE INVENTION

To achieve the above-mentioned object, in the cathode ray tube accordingto the present invention, the thickness of the glass and/or the radiusof curvature of a faceplate panel are constituted as follows.

(1) Assuming a thickness of a screen effective area of a faceplateportion along the tube axis direction as tc at a central sectionthereof, as tx at a peripheral section along the major axis directionand as td at a peripheral section along the diagonal direction, therelationship tc>td>tx is established. Due to such a constitution, thebrightness of a displayed image at the peripheral section is enhancedand, at the same time, the radius of curvature of the shadow maskbetween tire central section and the peripheral section along thediagonal direction can be made small, so that it becomes possible toprevent the lowering of the molding retaining strength of the curvedsurface.

(2) Assuming a thickness of a screen effective area of a faceplateportion along the tube axis direction as tc at a central section thereofand as tx at a peripheral section along the major axis direction, therelationship tc>tx is established, and assuming an equivalent radius ofcurvature of a screen effective area of an inner surface of thefaceplate portion as Rix in the major axis direction and as Rid in thediagonal direction, the relationship Rix>Rid is established. Due to sucha constitution, the brightness of a displayed image at the peripheralsection is enhanced and, at the same time, the lowering of the moldingretaining strength of the curved surface of the shadow mask between thecentral section and the peripheral section along the diagonal directioncan be prevented.

(3) Assuming a thickness of a screen effective area of a faceplateportion along the tube axis direction as tc at a central sectionthereof, as tx at a peripheral section along the major axis directionand as ty at a peripheral section along the minor axis direction, therelationship tc>ty>tx is established. Due to such a constitution, thebrightness of a displayed image at the peripheral section is enhancedand, at the same time, the lowering of the mechanical strength of anevacuated glass bulb (panel portion) at the peripheral section along theminor axis direction can be prevented. Further, the radius of curvatureof the shadow mask at the peripheral section along the minor axisdirection becomes small so that the lowering of the mechanical strengthof the shadow mask can be prevented.

(4) Assuming a thickness of a screen effective area of a faceplateportion along the tube axis direction as tc at a central section thereofand as tx at a peripheral section along the major axis direction, therelationship tc>tx is established, and assuming an equivalent radius ofcurvature of the screen effective area of an inner surface of thefaceplate portion as Rix in the major axis direction and as Riy in theminor axis direction, the relationship Rix>Riy is established. Due tosuch a constitution, the brightness of a displayed image at theperipheral section is enhanced and, at the same time, the lowering ofthe mechanical strength of an evacuated glass bulb (panel portion) andthe shadow mask at the respective peripheral sections along the minoraxis direction can be prevented.

(5) Assuming a thickness of a screen effective area of a faceplateportion along the tube axis direction as tc at a central sectionthereof, as ty at a peripheral section along the minor axis directionand as td at the peripheral section along the diagonal direction, therelationship tc>td>ty is established, and assuming an equivalent radiusof curvature of the screen effective area of an inner surface of thefaceplate portion as Riy in the minor axis direction and as Rid in thediagonal direction, the relationship Rid>Riy is established, andassuming an equivalent radius of curvature of a screen effective area ofan outer surface of the faceplate portion as Roy in the minor axisdirection and as Rod in the diagonal direction, the relationship Rod>Royis established. Also, due to such a constitution, the brightness of adisplayed image at the peripheral section is enhanced and, at the sametime, the lowering of the mechanical strength of an evacuated glass bulb(panel portion) and the shadow mask at the respective peripheralsections along the minor axis direction can be prevented.

Further, in addition to the above-mentioned constitutions, whennecessary, the hole transmittivity of a black matrix for formingphosphor dots or stripes on a phosphor screen is defined in a givenrange.

According to the above-mentioned constitutions of the present invention,the brightness of the displayed image at the peripheral section of thefaceplate can be made to match the brightness of the displayed image atthe central section so that when electron beams are irradiated to thephosphor screen on the inner surface of the faceplate, the brightness ofthe image displayed on the phosphor screen is prevented from beinglowered at the peripheral section, whereby the uniformity of thebrightness of the whole surface of the display screen can be maintained.Further, since it is no longer necessary to excessively increase thehole transmittivity of the black matrix at the periphery of the screen,a color cathode ray tube which does not give rise to a largedeterioration of the resolution at the periphery of the screen and whichexhibits a favorable color purity can be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an axial cross-sectional view showing the constitution of oneembodiment of a cathode ray tube according to the present invention.

FIG. 2 is a cross-sectional view of an embodiment of a panel faceplateaccording to the present invention.

FIG. 3 is a sectional view showing an equivalent radius of curvaturewhen a panel faceplate portion is aspherical.

FIG. 4 is a diagram showing a partial perspective view of theconstitution of a screen effective area of a faceplate portion of apanel in a color cathode ray tube of the present invention.

FIG. 5 is a diagram of a black matrix having a dot type phosphor screen.

FIG. 6 is a diagram of a black matrix having a stripe type phosphorscreen.

FIG. 7 is a diagram of a dot type shadow mask.

FIG. 8 is a plan view of a panel illustrating the present invention.

FIGS. 9A and 9B are cross-sectional views of a panel according toanother embodiment of the present invention.

FIGS. 10A and 10B are cross-sectional view of a panel according to stillanother embodiment of the present invention.

FIG. 11 is a diagrammatic cross-sectional view of a panel formedaccording to a conventional technique.

BEST MODE FOR CARRYING OUT THE INVENTION

Various embodiments of the present invention will be explainedhereinafter in conjunction with the accompanying drawings.

FIG. 1 is a cross-sectional view showing the constitution of oneembodiment of a cathode ray tube according to the present invention, andit illustrates an example in which the cathode ray tube is constitutedas a color cathode ray tube.

In FIG. 1, the cathode ray tube has a panel portion 1, a faceplate 1A, apanel skirt portion 1B, a neck portion 2, a funnel portion 3, a phosphorfilm 4, a shadow mask 5, an inner magnetic shield 6, a deflection yoke7, a purity adjusting magnet 8, a center beam static convergenceadjusting magnet 9, a side beam static convergence adjusting magnet 10,an electron gun 11, which generates electron beam 12.

A glass envelope (bulb) which constitutes the color cathode ray tube iscomprised of the panel portion 1, which is arranged at the front sideand has a large diameter, a thin elongated neck portion 2, whichaccommodates the electron gun 11, and the funnel portion 3, which isformed in an approximately funnel shape and connects the panel portion 1and the neck portion 2. The panel portion 1 includes the faceplate 1A,which constitutes a front surface, and a skirt portion 1B, which isconnected to the funnel portion. The phosphor film 4 is formed on aninner surface of the faceplate 1A by coating, and the shadow mask 5 ismounted such that the shadow mask 5 is arranged to face the phosphorfilm 4 in an opposed manner. The inner magnetic shield 6 is mountedinside of a connecting portion of the panel portion 1 and the funnelportion 3, while the deflection yoke 7 is arranged at the outside of theconnecting portion of the funnel portion 3 and the neck portion 2. Threeelectron beams (only one beam shown in the drawing) which are irradiatedfrom the electron gun 11 are deflected in given directions by thedeflection yoke 7 and impinge on the phosphor film 4 through the shadowmask 5. Here, the purity adjusting magnet 8, the center beam staticconvergence adjusting magnet 9 and the side beam static convergenceadjusting magnet 10 are arranged in parallel outside the neck portion 2.

The operation of the color cathode ray tube having such a constitution,that is, the image displaying operation, is the same as the imagedisplay operation of a known color cathode ray tube, and, hence, such anexplanation is omitted.

FIG. 2 is a cross-sectional view showing the constitution of thefaceplate 1A portion of the panel portion 1 in the color cathode raytube of the embodiment shown in FIG. 1.

In FIG. 2, numeral 1A(1) indicates the faceplate inner surface, numeral1A(2) indicates the faceplate outer surface, tc indicates the thicknessof the central section of the faceplate 1A, ta indicates the thicknessof the peripheral section of the faceplate 1A, Ri indicates the radiusof curvature of the faceplate inner surface 1A(1), and Ro indicates theradius of curvature of the faceplate outer surface 1A(2). Further, withrespect to components which are identical with the components shown inFIG. 1, the same symbols are employed. Here, the thicknesses tc, ta ofthe central and peripheral sections of the faceplate 1A indicate theshortest distances between the faceplate inner surface 1A(1) and thefaceplate outer surface 1A(2) at respective sections. Further, since theradii of curvature of the inner and outer surfaces of the faceplate areusually considerably larger than the thickness of the faceplate, tirethickness ta at the peripheral section of the faceplate 1A may bereplaced by the distance between the faceplate inner surface 1A(1) andthe faceplate outer surface 1A(2) which is parallel to the tube axisdirection.

As shown in FIG. 2, with respect to the faceplate 1A of this embodiment,the relationship between the radius of curvature Ri of the faceplateinner surface 1(1) and the radius of curvature Ro of the faceplate outersurface 1A(2) is set to Ro≦Ri+tc, and the thickness ta of the peripheralsection of the faceplate can be set to a value approximately equal to orslightly thinner than the thickness tc of the central section.

The faceplate 1A of the panel portion 1 of this embodiment is designedthrough following steps.

First of all, at step Si, the radius of curvature Ro of the faceplateouter surface 1A(2) of the faceplate 1A is set.

Then, at step S2, the thickness tc of the central section of thefaceplate 1A is set.

Thereafter, at step S3, the thickness ta of the peripheral section offaceplate 1A is set such that the thickness ta is substantially equal toor smaller than the thickness tc of the central section set at thepreceding step S2.

Subsequently, at step S4, the radius of curvature Ri of the faceplateinner surface 1A(1), is set which satisfies the thickness tc of thecentral section and the thickness ta of the peripheral section which areset, at the preceding steps S2 and S3.

Then, at step S5, given strength calculations are performed with respectto the faceplate 1A of the panel portion 1 having the radius ofcurvature Ri of the faceplate inner surface 1A(1) and the radius ofcurvature Ro of the faceplate outer surface 1A(2), which arerespectively set at the preceding steps S4 and S5.

Subsequently, at step S6, if it is determined that the result of thestrength calculation which is performed at the preceding step S5 isequal to or more than a given value at the preceding step S5, the designof the faceplate 1A of the panel portion 1 having the radius ofcurvature Ri of the faceplate inner surface 1A(1) and the radius ofcurvature Ro of the faceplate outer surface 1A(2) is completed. On theother hand, if it is determined that the result of the strengthcalculation is equal to or less than the given value, the processingreturns to the preceding step S3 and the processings at step S3 andsucceeding steps are performed again.

The color cathode ray tube having the faceplate 1A of the panel portion1, which is obtained in the above-mentioned manner, is constituted suchthat the thickness ta of the peripheral section of the faceplate 1Abecomes substantially equal to or thinner than the thickness tc of thecentral section. Accordingly, it becomes possible to set the lighttransmittivity in the periphery of the screen to be substantially equalto or less than the light transmittivity at the center of the screen,and, hence, the brightness can be made substantially uniform over thewhole screen.

Although the above constitution has been explained on the premise thatthe faceplate inner surface or the faceplate outer surface is spherical,it is needless to say that the above constitution is also applicable toa case in which the inner surface or the outer surface of the faceplateis aspherical.

FIG. 3 is a view which shows the equivalent radius of curvature when thefaceplate portion of the panel is aspherical. As illustrated in FIG. 3,when the faceplate portion is aspherical, the equivalent radius ofcurvature RE is defined by a following equation based on therelationship between the equivalent radius of curvature RE and a fallamount Z from the center of the faceplate.

RE=(Z ² +d ²)/2Z

An advantage of the aspherical panel lies in that the difference in thethickness of the panel on the diagonal axis, on the major axis and onthe minor axis can be independently determined with respect to arequired predetermined value of brightness.

FIG. 4 is a perspective view which shows the constitution of the screeneffective area of the faceplate 1A portion of the panel portion 1 in thecolor cathode ray tube of the embodiment of the present invention.

In FIG. 4, x indicates an axis in the horizontal direction on the screen(major axis), y indicates an axis in the vertical direction on thescreen (minor axis), z indicates an axis in the direction which passesthrough the center of the screen and is perpendicular to both the majoraxis and the minor axis (tube axis direction), tc indicates the glassthickness of the central section in the tube axis direction, txindicates the glass thickness of the peripheral section along the majoraxis in the tube axis direction, ty indicates the glass thickness of theperipheral section along the minor axis in the tube axis direction, tdindicates the glass thickness of the peripheral section along thediagonal axis in the tube axis direction, Rix indicates the equivalentradius of curvature of the inner surface in the major axis direction,Riy indicates the equivalent radius of curvature of the inner surface inthe minor axis direction, Rid indicates the equivalent radius ofcurvature of the inner surface in the diagonal axis direction, Roxindicates the equivalent radius of curvature of the outer surface in themajor axis direction, Roy indicates the equivalent radius of curvatureof the outer surface in the minor axis direction, and Rod indicates theequivalent radius of curvature of the outer surface in the diagonal axisdirection.

With respect to the constitution of the screen effective area of thefaceplate 1A portion of the panel portion 1 in the color cathode raytube of this embodiment, the size relationship among the glass thicknesstc of the central section, the glass thickness tx of the peripheralsection along the major axis direction, the glass thickness ty of theperipheral section along the minor axis direction and the glassthickness td of the peripheral section along the diagonal axis directionis set to a relationship tc>td>tx, a relationship tc>td>ty or arelationship tc>ty>tx. Here, the constitution of the screen effectivearea of the faceplate 1A portion of the panel portion 1 may beconstituted to have two or more of these relationships.

To set the size relationship of the glass thickness of respectivesections in the above-mentioned manner, the size relationship among theequivalent radius of curvature Rix of the inner surface along the majoraxis direction, the equivalent radius of curvature Riy of the innersurface along the minor axis direction and the equivalent radius ofcurvature Rid of the inner surface along the diagonal axis direction isset to a relationship Rix>Rid, a relationship Rix>Riy or a relationshipRid>Riy. Further, the size relationship among the equivalent radius ofcurvature Rox of the outer surface along the major axis direction, theequivalent radius of curvature Roy of the outer surface along the minoraxis direction and the equivalent radius of curvature Rod of the outersurface along the diagonal axis direction is set to a relationshipRox>Roy or a relationship Rod>Roy. Here, the size relationship of theglass thickness of respective sections may be constituted to have two ormore of these relationships.

With respect to the color cathode ray tube having the faceplate 1Aportion of the panel portion 1 which is obtained in the above-mentionedmanner, since the equivalent radius of curvature Rid of the innersurface along the diagonal direction is smaller than the equivalentradius of curvature Rix of the inner surface along the major axisdirection, corresponding to the curved shape of the inner surface of thefaceplate, the radius of the curvature of the shadow mask, which isarranged to face the phosphor screen formed on the inner surface of thefaceplate in an opposed manner and performs color selection, is also setto be small between the center and the peripheral section along thediagonal direction. The cathode ray tube having such a constitution canexhibit an increased molding retaining strength of the curved surface ofthe shadow mask between the peripheral section along the diagonaldirection which is most distant from the center of the shadow mask andthe center so that the curved surface is hardly deformed, whereby theelectron beams which pass through apertures of the shadow mask cannormally impinge on the phosphor screen (can perform normal colorselection) thus preventing the color purity of the displayed image frombeing deteriorated.

Here, the glass thickness td of the peripheral section along thediagonal direction is set to a value in a range which is smaller thanthe glass thickness tc of the central section and is larger than theglass thickness tx of the peripheral section in the major axis directionor the glass thickness ty of the peripheral section in the minor axisdirection; and, hence, the brightness of the display image can beenhanced and the uniformity of the brightness over the entire surfacecan be maintained.

Further, since the equivalent radius of curvature Riy of the innersurface along the minor axis direction is set to be smaller than theequivalent radius of curvature Rix of the inner surface along the majoraxis direction or the equivalent radius of curvature Rid of the innersurface along the diagonal axis direction, corresponding to the curvedshape of the inner surface of the faceplate, the radius of the curvatureof the shadow mask, which is arranged to face the phosphor screen formedon the inner surface of the faceplate in an opposed manner and performscolor selection, is also set to be small at the peripheral section alongthe minor axis direction. The cathode ray tube having such aconstitution can exhibit an increased mechanical strength of the curvedsurface at the peripheral section along the minor axis direction, whichis liable to be easily influenced by vibrations or an impact, such asfrom dropping or the like, of the shadow mask, so that the curvedsurface is hardly deformed, whereby the electron beams which passthrough the apertures of the shadow mask can normally impinge on thephosphor screen (can perform the normal color selection), thuspreventing the color purity of the displayed image from beingdeteriorated.

Here, since the glass thickness ty of the peripheral section along theminor axis direction is set to be thicker than the glass thickness tx ofthe peripheral section along the major axis direction, or since theequivalent radius of curvature Riy of the inner surface in the minoraxis direction is set to be smaller than the equivalent radius ofcurvature Rix of the inner surface in the major axis direction or theequivalent radius of curvature Rid of the inner surface in the diagonaldirection, the concentration of the stress strain caused by the vacuumin the glass bulb (panel portion) at the peripheral section along theminor axis direction can be alleviated so that the deterioration of themechanical strength of the glass bulb (panel portion) can be prevented.

Further, in addition to setting the relationship in which the equivalentradius of curvature Riy of the inner surface in the minor axis directionis made smaller than the equivalent radius of curvature Rix of the innersurface in the major axis direction or the equivalent radius ofcurvature Rid of the inner surface in the diagonal axis direction, theequivalent radius of curvature Roy of the outer surface in the minoraxis direction is set to be smaller than the equivalent radius ofcurvature Rox of the outer surface in the major axis direction or theequivalent radius of curvature Rod of the outer surface in the diagonalaxis direction. Accordingly, the concentration of the stress straincaused by the vacuum in the glass bulb (panel portion) at the peripheralsection along the minor axis direction can be alleviated, so that thedeterioration of the mechanical strength of the glass bulb (panelportion) can be prevented.

As a method which compensates for the difference in brightness betweenthe center and the periphery, a method which sets the holetransmittivity of the black matrix (BM) at the periphery larger thanthat at the center is considered.

FIG. 5 is a diagram of a black matrix having a dot type phosphor screen,while FIG. 6 is a diagram of a black matrix having a stripe typephosphor screen.

Here, the BM hole transmittivity is a rate of portions which lack ingraphite 4BM as shown in FIG. 5 and FIG. 6, that is, a rate which allowslight to pass through the portions. Here, PD denotes the dot pitch,which is an interval between phosphor bodies of the same color. However,when a panel which has a glass thickness that is thicker at theperiphery than at the center is used in the same manner as aconventional panel, unless the BM hole transmittivity at the peripheryis increased by equal to or more than 10%, it is difficult to obtain anapproximately uniform brightness over the center and the periphery. Asmeans to increase the BM hole transmittivity at the periphery of thescreen without sacrificing the landing tolerance, there exists a methodwhich increases the dot pitch at the periphery of the screen relative tothat at the center of the screen. However, when the dot pitch is madeexcessively large at the periphery, it gives rise to a deterioration ofthe resolution at the periphery of the screen. Further, when the holetransmittivity is increased at the periphery, since the electron beamswhich have passed through the apertures of the shadow mask cannot bemade sufficiently larger than the holes of the BM, it gives rise to aloss of beams, which is a phenomenon in which the electron beams fail tocover the hole portions. To prevent such loss of beams, thetransmittivity of the shadow mask may be increased. However, this givesrise to a problem that the strength of the shadow mask is lowered.

FIG. 7 is a diagram of a dot type shadow mask. Here, the transmittivityof the shadow mask is a rate of the area of the shadow mask apertures 51as shown in FIG. 7.

The present invention is characterized by the fact that the thickness ofthe panel is set such that the thickness of the periphery of the panelis equal to the thickness of the center, or is thinner than the centerand the BM hole transmittivity is defined so as to be based on arelationship with respect to the thickness of the panel, so that thedifference in brightness between the center and the periphery can bemade small, while ensuring the landing tolerance.

Here, even when the thickness of the panel and the BM holetransmittivity are made substantially equal between the center and theperiphery, due to reasons such as (1) the weight of the phosphor dotsbecomes smaller at the periphery of the screen than the center of thescreen, (2) the reflectance of a metal back which reflects Light fromthe phosphor is decreased at the periphery of the screen and the like,the brightness of the periphery becomes lower than that of the center.Accordingly, there arises a case in which, even when the thickness ofthe panel is slightly thin at the periphery, it is necessary to increasethe BM hole transmittivity at the periphery. Even in such a case, the BMhole transmittivity of the periphery of the screen relative to thecenter of the screen can be set to be equal to or less than 110%, andfurthermore, the BM hole transmittivity can be set to be equal to orless than 105% depending on the balance between the thickness and thedifference in brightness. According to the most preferred embodiment ofthe present invention, the BM hole transmittivity of the periphery isset to be lower than that of the center. Further, when the BM holetransmittivity at the periphery of the screen is set to be equal to ormore than 70% of the BM hole transmittivity at the center of the screen,the brightness ratio between the periphery of the screen and the centerof the screen can be enhanced. By setting the BM hole transmittivity atthe periphery of the screen to be equal to or more than 90% of the BMhole transmittivity at the center of the screen, the brightness ratiowith respect to the center of screen can be further enhanced. Due tosuch a constitution, the brightness difference between the center andthe periphery can be eliminated, and it becomes possible to attain thenecessary landing tolerance at the periphery. Here, when the BM holetransmittivity at the periphery of the screen is set to be equal to orless than 110% of the BM hole transmittivity at the center of thescreen, the dot pitch at the periphery also can be set to be equal to orless than 110% of the dot pitch at the center, and, hence, thedeterioration of the resolution at the periphery is not so noticeable.In the same manner, provided that the BM hole transmittivity at theperiphery of the screen is set to be equal to or less than 105% of theBM hole transmittivity at the center of the screen, the dot pitch at theperiphery also can be set to be equal to or less than 105% of the dotpitch at the center, and, hence, the deterioration of the resolution atthe periphery is hardly noticeable. Further, since it is no longernecessary to excessively increase the transmittivity of the shadow maskat the periphery or it becomes possible to decrease the transmittivityof the shadow mask at the periphery, the strength of the shadow mask canbe ensured.

Provided that the BM hole transmittivity at the periphery of the screenis set to be equal to or less than 110% of the BM hole transmittivity atthe center of the screen, it is also possible to restrict thetransmittivity at the periphery of the shadow mask to be equal to orless than 110% of the transmittivity at the center of the shadow mask.To take the tolerance of the strength of the shadow mask intoconsideration, it is preferable to set the transmittivity of the shadowmask at the periphery of the screen lower than that of thetransmittivity of the shadow mask at the center of the screen.

FIG. 8 is a plan view of the panel provided for the present invention.Here, as shown in FIG. 8, the faceplate peripheral section is aperipheral area which corresponds to phosphor dots or stripes of thephosphor film 4, which is formed on the faceplate inner surface 1A(1) bycoating, that is, a peripheral section of an effective screen 111 onwhich an image is displayed.

As shown in FIG. 8, the effective screen periphery can be represented bya periphery 112 along the diagonal direction, a periphery 113 along themajor axis direction, and a periphery 114 along the minor axisdirection. In general, the periphery which generates the most crucialproblem with respect to a difference in brightness between the peripheryand the center of the screen is the periphery 112 along the diagonaldirection. Then, the periphery along the major axis direction 113 andthe periphery 114 along the minor axis direction follow sequentially. Inreality, the thickness of the panel, the BM hole transmittivity and theshadow mask transmittivity of the above-mentioned respective portionsmay be set in response to a request for a brightness distribution of aproduct.

In a color cathode ray tube for a display monitor to be used at acomputer terminal or the like, in many cases, so-called tint (havingtransmittivity of 56.8% at a late thickness of 10.6 mm and having EIAJstandard transmittivity when measured with light having wavelength of546 nm) is used as a panel glass for increasing the contrast, and a darktint (transmittivity of 46% at a plate thickness of 10.6 mm and havingEIAJ standard transmittivity when measured with light of wavelength 546ma) is used to further enhance the contrast. The present invention isparticularly effective in a case where glass having such lowtransmittivity is used.

Further, in a high definition tube which sets the dot pitch at thecenter to be equal to or less than 0.26 imu, the landing tolerance ofthe electron beams to the phosphor is small at the periphery of thescreen, so that it is difficult to increase the BM hole transmittivityat the periphery. Accordingly, the present invention is particularlyapplicable to such a color cathode ray tube for a display monitor.

Further, the difference in brightness between the center and theperiphery of the screen is liable to be noticeable in a large-sizedtube. The present invention is particularly effective in a large-sizedcolor cathode ray tube for a display monitor having an effective screensize in the diagonal direction which is set to be equal to or more than46 cm (19 inches).

An example in which the present invention is applied to a color cathoderay tube for a display monitor having a size of 19 inches will bedescribed. Here, the panel base is a tint.

center diagonal periphery panel thickness 12.5 mm 11.3 mm BM holetransmittivity 42.4% 39.8% shadow mask transmittivity 17.6% 17.1% dotpitch 0.26 mm 0.27 mm

To enhance the mechanical strength, a shadow mask which is applied tothe 19 inch color cathode ray tube for a display monitor is shaped tohave a given curved surface, and the weight of an apertured portionafter coating a blackened film on a surface thereof is set to be equalto or less than 48 g. Due to such a constitution, it becomes possible toprevent the deformation of the curved surface of the peripheral sectionof the shadow mask, which may be caused by dropping the color cathoderay tube or an impact to cause vibrations or the like.

Subsequently, FIGS. 9A and 9B are partial cross-sectional views of apanel according to another embodiment of the present invention, whereinFIG. 9A is a cross-sectional view in the diagonal direction of thescreen and FIG. 9B is a cross-sectional view in the minor axis directionof the screen.

With respect to the brightness ratio between the periphery and thecenter of the screen, the periphery 114 along the minor axis hardlyproduces a problem. On the other hand, with respect to the strength ofthe shadow mask, the periphery along the minor axis shows the leasttolerance. The strength of the shadow mask can be increased by providinga curvature. The curved surface of the shadow mask is stronglyinfluenced by the curvature of the inner surface of the panel. From thispoint of view, it is preferable to select a small radius of curvaturefor the inner surface of the panel. That is, the panel thickness at thediagonal periphery of the screen is set to be smaller than the panelthickness at the center, and the panel thickness at the periphery of thescreen along the minor axis is set to be larger than the panel thicknessat the center, so that the brightness difference between the center andthe periphery can be decreased while maintaining the required shadowmask strength. This embodiment is shown in FIGS. 9A and 9B.

Even when the panel thickness at the center and the panel thickness atthe periphery are substantially equal, the brightness difference betweenthe center and the periphery of the screen can be decreased compared toa conventional example. In this case, it is preferable to set the BMhole transmittivity at the periphery of the screen to be equal to ormore than 70% of the BM hole transmittivity at the center of the screen,and it is more preferable to set the BM hole transmittivity at theperiphery of the screen to be equal to or more than 90% of the BM holetransmittivity at the center of the screen. It is further preferable toset the BM hole transmittivity at the periphery to be larger than the BMhole transmittivity at the center. Even in such a case, it becomespossible to improve the brightness ratio compared to a conventional casein which the panel thickness becomes large at the diagonal periphery,and even when the hole transmittivity at the diagonal periphery is equalto or less than 110% of the hole transmittivity at the center, thebrightness difference can be restricted to a practical brightnessdifference.

Provided that the hole transmittivity at the periphery of the screen isset to be equal to or less than 110% of the hole transmittivity at thecenter of the screen, the dot pitch at the periphery of the screen canbe suppressed so as to be equal to or less than 110% of the dot pitch atthe center of the screen, so that the deterioration of the resolution atthe periphery of the screen is not so noticeable. In the same manner,provided that the hole transmittivity at the periphery of the screen isset to be equal to or less than 105%, it becomes possible to suppressthe dot pitch at the periphery of the screen to equal to or less than105% of the dot pitch at the center of the screen, so that thedeterioration of the resolution at the periphery of the screen becomeshardly noticeable.

FIGS. 10A and 10B are cross-sectional views of a panel according tostill another embodiment of the present invention, wherein FIG. 10A is across-sectional view along the diagonal direction of the screen and FIG.10B is a cross-sectional view along the minor axis direction of thescreen.

When the outer surface of the panel is flat, according to the presentinvention, the inner surface of the panel has a radius of curvature inthe inverse direction with respect to the diagonal direction as shown inFIG. 10A. Even in such a case, by setting the radius of curvature Rid inthe inverse direction in the diagonal direction and by setting theradius of curvature Riy in the normal direction with respect to theminor axis direction, the brightness ratio between the center and thediagonal periphery can be decreased while maintaining the strength ofthe shadow mask.

When the inner surface of the panel is flat, by giving a propercurvature to the outer surface of the panel, the brightness ratiobetween the center of the screen and the diagonal periphery of thescreen can be decreased.

According to the present invention, the brightness difference betweenthe center of the screen and the periphery of the screen can be reducedwhile maintaining the landing tolerance at the periphery of the screen.

Further, according to the present invention, the brightness differencebetween the center of the screen and the periphery of the screen can bereduced without lowering the mechanical strength of the shadow mask andthe glass bulb.

As has been described heretofore, the color cathode ray tube accordingto the present invention is suitable for use in a color television sethaving a large screen or a high definition color display monitor.

What is claimed is:
 1. A color cathode ray tube comprising a glass bulbwhich includes a panel portion having a faceplate portion having aphosphor screen on an inner surface thereof, a neck portion which mountsan electron gun inside thereof, and a funnel portion which connects thepanel portion and the neck portion, wherein a thickness of a screeneffective area of the faceplate portion along a tube axis direction istc at a central section thereof, is tx at a peripheral section thereofalong a major axis direction and is td at a peripheral section thereofalong a diagonal direction, and wherein a relationship tc>td>tx isestablished.
 2. A color cathode ray tube comprising a glass bulb whichincludes a panel portion having a faceplate portion having a phosphorscreen on an inner surface thereof, a neck portion which mounts anelectron gun inside thereof, and a funnel portion which connects thepanel portion and the neck portion, wherein a thickness of a screeneffective area of the faceplate portion along a tube axis direction istc at a central section thereof and is tx at a peripheral sectionthereof along a major axis direction, and a relationship tc>tx isestablished, and wherein an equivalent radius of curvature of the screeneffective area of the inner surface of the faceplate portion is Rix inthe major axis direction and is Rid in a diagonal direction, and arelationship Rix>Rid is established.
 3. A color cathode ray tubecomprising a glass bulb which includes a panel portion having afaceplate portion having a phosphor screen on an inner surface thereof,a neck portion which mounts an electron gun in the inside thereof, and afunnel portion which connects the panel portion and the neck portion,the color cathode ray tube having a shadow mask mounted with respect tothe phosphor screen on the inner surface of the faceplate portion,wherein a thickness of a screen effective area of the faceplate portionalong a tube axis direction is tc at a central section thereof, is tx ata peripheral section thereof along a major axis direction and is ty at aperipheral section thereof along a minor axis direction, and wherein arelationship tc>ty>tx is established.
 4. A color cathode ray tubecomprising a glass bulb which includes a panel portion having afaceplate portion having a phosphor screen on an inner surface thereof,a neck portion which mounts an electron gun inside thereof, and a funnelportion which connects the panel portion and the neck portion, wherein athickness of a screen effective area of the faceplate portion along atube axis direction is tc at a central section thereof and is ty at aperipheral section thereof along a minor axis direction and is td at theperipheral section along a diagonal direction, and a relationshiptc>td>ty is established, wherein an equivalent radius of curvature of ascreen effective area of the inner surface of the faceplate portion isRiy in the minor axis direction and is Rid in the diagonal direction,and a relationship Rid>Riy is established, and wherein an equivalentradius of curvature of a screen effective area of an outer surface ofthe faceplate portion is Roy in the minor axis direction and is Rod inthe diagonal direction, and a relationship Rod>Roy is established.